It is known that in the medical diagnostic applications there is frequently a need for portable instruments which are easy to handle, in such a way as to allow a direct use of the instrument (detector) on the patient and a display of the images with dedicated units positioned close to the detector. These type of detectors are characterised by a limited measuring area and a relatively light weight.
This type of application finds a technical justification linked to the fact that the overall weight of the detector may only be reduced by reducing the measuring area and consequently the use of portable detectors may find a valid use, for example, in operating rooms and in radioguided surgery, as well as in the diagnosis of small organs. The separation between detector and control/display unit is often necessary to reduce the weight of the entire detector, since otherwise it would not be easy to handle in use.
Typically, the weight of these detectors is due mainly to the materials for shielding against external radiation (shielding of the scintillation structure and collimator) which must not reach the measuring surface and typically the weight is about 1-2 kg for the most advanced detectors, having a small measuring area (5 cm×5 cm). Clearly, the component linked to the use of the electronic equipment also affects the final dimensions and, consequently, the actual possibility of making the detector made in this way easy to handle.
Therefore, the above-mentioned portable detectors prevent the display of the images directly on the same structure handled by the operator. A small device, having a small area and which can be easily handled, may be positioned directly on the organ in question, which is extremely difficult to achieve with a large detector. Reducing the organ-detector distance also has a considerable affect on the spatial resolution of the devices for diagnostic purposes.
In common practice, the use of large detectors sometimes allows for adjustments to the organ-detector position, to be performed only after some preliminary acquisitions and forcing the operator to make successive positionings of the detector on the area to be analysed. In practice, the need to separate the measuring units from the control and display unit, even if only limited to systems with large areas, finds a logic in the type of investigation in which the detector is much larger than the organ of the patient to be analysed whilst, on the other hand, it is extremely critical when the measuring area is small compared with the area where the detector is to be positioned to search for any diseases and which therefore needs rapid successive explorations. The advantage in the operating room appears very evident where the exploration of areas of tissue with a small detector necessarily needs preliminary measurements for the correct positioning of the measuring area on the part in question or on the organ (colon, breast, thyroid and parathyroids, etc.)
In the case of detectors with small areas, where the detector is, in any case, separated from the control and display unit, for obvious reasons of reducing the weight of the entire device, the need to position the detector with respect to the physical area to be investigated results in the need for the operator to identify diagnostic details of the image observed on an external monitor, far from the corresponding investigation area, with the consequent need to apply inevitable approximations with respect to that which is displayed, not having an immediate correspondence between the physical area of the display and that of the detector.
A portable measuring instrument is also known, from the United States patent US2011/0208049, which has a display screen integrated in the detector itself.
However, this instrument has measuring elements (made from semiconductors, in particular CdZnTe) with very large dimensions (3 mm×3 mm) which considerably penalise the achievement of acceptable spatial resolutions.
More specifically, patent US2011/0208049 indicates a total investigation area (for the system known as “Microimager”) which ranges from 3 inches×3 inches up to 5 inches×5 inches. Developing these measuring areas, the minimum number of measuring elements for the smallest device, using 3 mm×3 mm pixels, would be 625. Every element of CdZnTe is connected with a single pre-amplifier using a series of ASIC chips known as “RENA”, each of which can control up to a maximum of 32 signals. In practice, at least twenty RENA chips would be required to control all the signals necessary for the operation of the gamma camera. Considering the dimensions of these chips, which are commercially produced in the updated version of 36 simultaneous signals per single chip, the volume necessary for packaging the chips and their control card appears very high and not easily suited to the desired characteristics of compactness and ease of handling. Moreover, since every pre-amplifier develops an absorption of at least 5 mW per channel, as indicated by the latest model produced, the total consumption would be equal to at least 3 W. Moreover, in order to control 32-36 signals, each RENA chip is combined with a single ADC (analogue-digital converter) with an average consumption of approximately 100 mW. Consequently, 20 RENA chips require at least as many ADCs, with a resulting average consumption of at least another 2 W. The development of the RENA-3 cards results in an integrated card with 4 RENA chips mounted on board for simultaneously controlling 4 blocks with 4 ADCs, for a total of 144 channels. Each ADC is linked to the use of a FPGA, the average consumption of which may be estimated to be approximately 0.5 W. Consequently, the consumption linked to the use of 5 cards with 4 RENA chips on board is approximately at least 2.5 W. The total estimated for these electronics is therefore 7.5 W, without considering the other consumptions linked to other components (display, microprocessor, etc.).
In order to operate the 625 elements at least 5 complete cards of ADCs would be needed. The dimensions of the single RENA card with 4 integrated chips is approximately 20 cm×6 cm, with a thickness of at least 1-2 cm linked to the presence of components and connectors and the necessary presence of cooling fans for dissipating heat, required to reduce the temperature linked to the use of a multitude of cards which dissipate heat. In that situation, the absorption linked to the electronics for controlling the signals, without considering other consumptions, is very high (approx. 7-8 W) as well as certainly not providing small dimensions. In effect, the minimum area necessary to house the cards must be at least 20 cm×at least 6 cm, in addition to the positioning of the detector, the smallest dimension of which is approximately 7.5 cm×7.5 cm (3 inches×3 inches). For this reason, the dimensions of the outer container may not be less than 20 cm×10 cm×12-15 cm. The problems of high total absorption (approx. 8 W) and the total volume developed by the electronics required for the operation make it not very practical to achieve a device which is easy to handle (that is, compact and light). All of this with a total weight closely linked to the use of a collimator suitable for the diagnostic use. For a standard 24 mm collimator made of lead which can be adapted to the measuring area (7.5 cm×7.5 cm) and 2 mm holes, with 2 mm lead rings for cutting the non-parallel events which cross the partitions, a weight of not less than 600 grams may be assumed, to which it is necessary to add the 2 mm lateral shielding again made of lead for the measuring elements as well as the weight of the batteries necessary to operate the apparatus (with the above-mentioned consumptions very high) at least for a duration of 2 hours. Consequently, the weight of the device can easily exceed 2 kg and an estimated volume of 20 cm×10 cm×15 cm. From the data given in the above-mentioned patent, the absorption characteristics of the RENA cards, the number of which is strongly dependent on the number of CdZnTe pixels, are compatible with a total value of at least 8 W.
In the case of a larger area, as indicated in the text of the patent (and in particular in the case of a total measuring area of 5 inches×5 inches), 42×42 CdZnTe elements would be needed (a total of 1764 elements). The control of these elements requires 49 chips, if the new 36-channel RENA-3 is used. At least 11 cards would be necessary, fitted with a 4-channel ADC, if 4 RENA chips are mounted on each board. It would therefore be necessary to supply 1764 elements which absorb at least 5 mW each, bringing the absorption to approximately 9 W. The 49 cards with ADC on board would develop at least 5 W, whilst the absorption of the 11 cards with FPGA would consume another 5.5 W. It would all consume approximately 20 W and would have a total size of 20 cm×20 cm×15 cm.
Comparing the ratio between measuring area and overall volume, it may be considered that in the case of the above-mentioned patent, for the development of a measuring area of 3 inches×3 inches this value is approximately 1.9%, whilst in the case of a measuring area of 5 inches×5 inches this value is 2.7%
With reference to the performances which can be obtained, it is necessary to consider that the attempt to improve the spatial resolution in this type of detector would require reducing the size of the measuring elements and, consequently, increasing the number of pre-amplification channels of the RENA chips and of the ADCs. By way of an example, in order to reach a nominal intrinsic resolution of approximately 1 mm, the area of 3 inches×3 inches should have 5776 CdZnTe elements, so more than 160 RENA chips and more than 40 ADCs. This would all lead to a height of the detector of more than 80 cm, which clearly cannot be proposed as a technical solution. Moreover, the consumption in terms of absorption (65 W) would be extremely high for a small range device.
Similarly, the attempt to improve the ease of handling in this type of detector, which would therefore require reducing weights and dimensions of the detector, can only lead to the reduction of the electronics installed and therefore the reduction of the number of CdZnTe measuring elements. This, for the same total measuring area, considerably penalises the spatial resolution which can be obtained.
Thus, starting from the detector described in patent US2011/0208049, every attempt to improve the ease of handling of the detector would lead to a significant worsening of the spatial resolution whilst, on the other hand, every attempt to improve the spatial resolution of the detector would lead to a significant worsening of the ease of handling.
In other words, the teachings of US2011/0208049 make the size and consumption characteristics, which are fundamental elements for making a device compact and easy to handle, strongly dependent on the real spatial resolution dimensions which can be obtained. In order to reach acceptable resolution values this technology requires the use of particular electronic cards which are necessarily voluminous with respect to the requested performance and the total consumption of which also affects significantly the final weight (increase in the number of batteries, total weight of the system). It is evident that a device for which its volume increases due to the length necessary to achieve the optimum resolution does not represent a solution to the problem of making a device which is truly easy to handle, compact and light in weight.